Electrochemical recovery of metals from ionic liquids, that is, the recovery of metals from a solution by applying a sufficiently negative electric potential to the solution causing electrochemical reduction of a metal complex, and electrodeposition of a metal or metals at the cathode is a commonly used process. The metal complex may be formed, for instance, from the dissolution or leaching of an ore. As another example, the metal complex may be formed by the dissolution or corrosion of bulk metal from, or impurities within, a metallic object. The corrosion may also be actively accelerated by, for example, making the metal object the positive electrode (anode) in an electrochemical cell and oxidising its metal constituents electrochemically. The electrochemical reduction process, wherein the metal ions in the metal complex are reduced at the cathode from their charged states and deposited on the cathode in their zero valence states, may be called, for instance, electrolysis, electrowinning, electroplating, electrodeposition and electrorefining. In this specification, the process will be conveniently referred to as electrorecovery.
Ionic liquids are molten salts which typically melt below 100° C. The properties of these liquids vary widely, but some are very stable to oxidation and reduction and also have high conductivity. Ionic liquids with these properties are useful as solvents and electrolytes for numerous electrochemical applications including metal electrodeposition. Metal complexes in ionic liquid solution are almost always negatively charged (anionic) which inhibits their electrochemical reduction because coulombic forces repel them from negatively charged electrodes (cathodes) as reducing potentials are applied.
The electrochemical reduction of metal complexes in solution to form metals requires the metal complex to approach the negative electrode (cathode) closely. The electrical potential that must be applied to a cathode for reduction to occur varies depending on the identity of the metal complex but in most cases the electrode develops a negative surface charge when the potential is applied; that is, when the electrode is polarized. As a consequence of this surface charge, Coulombic forces affect the populations of ions near the surface of a polarized electrode.
In the vast majority of cases, cations such as metal ions dissolved in ionic liquids co-ordinate with the anion of the ionic liquid, for instance the bis(trifluoromethylsulfonyl)amide, abbreviated NTf2−, to produce negatively-charged complexes in solution. This is particularly true for metal complexes formed by dissolving metal salts in ionic liquids where the metal ion is attracted to, and often binds with, the negative ion (anion) of the ionic liquid. By contrast, metal salts in aqueous solution form positively charged metal aquo complexes.
When the metal complex is electrochemically reduced, during for example, electrodeposition, the complex diffuses towards the electrode where reduction takes place (the cathode) which itself is often negatively charged. The Coulombic force between the metal complex—which has a net negative charge—and the negatively-charged cathode, is repulsive. Moreover, as the metal complex approaches the cathode, the Coulombic force increases. However, the complex needs to approach the cathode to within a few nanometers (nm) before it will be electrochemically reduced at any appreciable rate. Thus, Coulombic forces repel almost all metal complexes in ionic liquids from a negatively charged electrode making their electrochemical reduction difficult and inefficient.
To overcome this repulsive force so that electrochemical reduction of the metal complex will occur at an appreciable rate, it is necessary to significantly increase the concentration of the metal complex and/or impart vigorous relative motion between the cathode and the solution by, for example, stirring. Neither step is commercially attractive.
Thus, there is a need to provide an electrolyte solution that removes, or at least reduces, the above described issues.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.